How many milliamps for this 12v spotlight

Hi all,

I'm trying to work out the load for the following spot lamp. I plan to use 5 of them, powered via a relay bank and need to work out the power supply rating (I will be working at 12 volts). There will be times in the sequence when all 5 are on at the same time.

Checking on my meter with a 12 volt supply it appears to be 110 milliamps per lamp but I'm not sure if I'm reading it correctly.

Your knowledgeable advice would be appreciated. :slight_smile:

MR16 GU5.3 Red LED Spotlight Bulbs 5W DC 12V-24V 120° Beam Angle GU5.3 Base Red Coloured Spot Light

John

Wattage is Voltage x Current so 5W and 12V is 417mA [math corrected].

These are LED bulbs specified to work over a wide input voltage range so I am not sure that this calculation applies. IF it applies, I get slightly over 2.08 amps for five bulbs.

You're right and my math was wrong.

Assuming it has a switchmode constant-current power supply, it will take less current from more voltage (with the same current going through the actual LEDs).

Switchmode power supplies are almost 100% efficient so as you increase the voltage, the current goes down. That's the voltage & current into the power supply. The voltage & current out of the power supply to the load remains the same.

Thank you for your replies.

I agree with the calculation being 417 mA. It also shows that in on-line calculators.

However, when I measure with my analogue and digital meters, they both read 105 mA, which is why I'm a bit confused. Perhaps, at 12 volts the lamps are not 5W ? At 12 volts they are more than bright enough for the display they will be used for. (The Red, Blue and Green lamps all read the same, give or take a couple of mA )

So, Do I go with the meter reading or the calculation?

Confused John :o

Wattage is Voltage x Current

These are LED bulbs specified to work over a wide input voltage range so I am not sure that this calculation applies.

The calculation always applies, it's basic physics!

I agree with the calculation being 417mA.
However, when I measure with my analogue and digital meters, they both read 105mA

Perhaps, at 12 volts the lamps are not 5W?

If they are drawing 105mA at 12V then they are consuming 1.26W, that's the power going in, not what comes out as light. Some will come out as heat.

At 12 volts they are more than bright enough for the display they will be used for.

Good, that's OK then.

Do I go with the meter reading or the calculation?

What your meter tells you is reality.
What is says on the packaging is is marketing bollocks.

PerryBebbington - Yes, the calculation properly applies to the input voltage and current. However, the bulb has been specified as 5W over a wide input range of voltages. Either the specification is wrong, or the bulb contains non-linear components (LED, power supply, etc.). The methods that most of us learned for analyzing circuits apply to linear (actually LTIV) circuits. For non-linear or poorly specified components, I would lean towards properly measured values.

Even the usual analysis of a LED is a linearization (fixed forward voltage drop, for example) of something that is non-linear in actuality.

DVDdoug - Thank you for the explanation.

Thanks again.

If I’ve understood correctly, I should go by the actual meter reading when calculating for the power supply rating. I’ll probably order one that’s double the power required to be safe.

John

If I've understood correctly, I should go by the actual meter reading when calculating for the power supply rating. I'll probably order one that's double the power required to be safe.

Agreed. They can print anything on the box, what you measure is what it actually takes. Yes, buy a PSU rated for more than you calculate, you should not run the PSU at its limit. Maybe +50% will be fine, even more is OK.

Let's keep things over board so 500 mA per light times 5 lights would be 2.5 Amps. Now as pointed out figure in aa advertising fudge factor on the stuff off the boat so you likely want a 12 Volt 5 Amp supply. You can find them on Amazon for about $12 USD and can likely find a 10 amp flavor for a few bucks more allowing for expansion. :slight_smile:

Ron

Hi,
Are your LED lamps DC or AC/DC?
I keep seeing these lamps specified a AC/DC.

Can you post a link to where you purchased them.
I keep seeing Amazon, and they provide little if any proper data.

Tom... :slight_smile:

Hello TomGeorge,

Here is the link. I've no idea if the spec is accurate but they do exactly what I need.

Regards
John

Hi,
Okay they are just raw LED lights to replace halogen units.
But 12V and 24V operation is strange.
Can you connect on to 24V and check the current and if the brightness is any higher?

Standard 50mm MR16 lamp shape, GU5.3 base. Perfect replacements for non-dimmable GU5.3/MR16 red halogen bulbs. Not to be installed on dimmers, sensors or timers.

Tom.... :slight_smile:

Hi Tom

Thanks for your response. I don't have a 24v supply available, so I can't aswer your question.

Regards

John

baxwalker:
If I've understood correctly, I should go by the actual meter reading when calculating for the power supply rating. I'll probably order one that's double the power required to be safe.

A good meter will usually tell you the real deal under the test conditions. You should also measure the voltage during the current measurement as the load being measured can pull down the supply voltage during the test. Calculations are based on assumptions that may or may not be valid at the test time, but can set bounding conditions.

I have seen marketing confuse watts with equivalent watts as incandescent.

wolframore:
I have seen marketing confuse watts with equivalent watts as incandescent.

Have you ever seen marketing that didn't confuse something? All marketing is bollocks.

I’m assuming they are 3 LED in series (per circuit) with a dropping resistor. 72 LEDS total, so 24 parallel. If they’re consuming about 100 mA total that’s only about 4 mA per LED at 12V. At least it’s not being overdriven. Assuming LEDs rated for 20mA it could be easily boosted higher. Since the bulbs are rated for 12-24v it’s probably using dropping resistors on each parallel circuit... I’m guessing about 750 Ohms. This would get you close to 5 watts at 24v. At 0.417 A that’s about 17mA each which could be achieved with above.

At 24V it’s not very efficient and will generate a lot of heat. Diodes are non linear with its kt/q ebersmoll relationship. It’s what makes them a pita to calculate.

I’m sure they just said can you design it so it works at 12-24v and they just did the simplest thing.. I’m sure those resistors are each dissipating almost 1/4 watt per circuit. That’s a lot of heat.

I have a marketing degree :o

wolframore:
I have a marketing degree :o

Oh!
Perhaps I've never seen any of your marketing then! :o

:wink: there are interesting areas in marketing like product development which is cool... now a days I’m more EET than MBA